One key guarantee for ris to improve the network capacity is the joint precoding. Different from the conventional precoding at the bs only, the joint precoding in ris-based wireless systems refers to the joint design of the beamforming vector at the bs and the phase shifts of the ris elements. Different RIS-based scenarios have been studied to maximize the capacity in the literature. Specifically, the authors in [1]} considered a scenario where one bs and one ris jointly serve a single user, which was an early attempt to realize the capacity enhancement by using low-cost and energy-efficient ris. In [2]}, the authors considered a multi-user scenario and maximized the sum-rate of all users. To obtain the cooperation gain, the multi-bs scenario was considered in [3]}, while the multi-ris case was investigated in [4]}, respectively. Besides, to satisfy the requirements of wideband system, the multi-carrier scenario has been discussed in [5]}.
